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United States Patent |
6,204,469
|
Fields, Jr.
,   et al.
|
March 20, 2001
|
Laser welding system
Abstract
The present invention relates generally to an improved laser welded work
piece, such as an automotive body panel, and a system and method for the
manufacture thereof. The invention is also directed to an improved system
for manufacturing the welded work piece including an improved laser welder
and a laser weld inspection device and system.
Inventors:
|
Fields, Jr.; Donald R. (Troy, OH);
Foley; James (Marysville, OH);
Morris; Darin (Urbana, OH);
Godsil; Frank (Marysville, OH)
|
Assignee:
|
Honda Giken Kogyo Kabushiki Kaisha (Tokyo, JP)
|
Appl. No.:
|
262248 |
Filed:
|
March 4, 1999 |
Current U.S. Class: |
219/121.6; 219/121.63; 228/103 |
Intern'l Class: |
B23K 026/00; B23K 026/42 |
Field of Search: |
219/121.63,121.64,121.83,121.6
228/103,105
73/866,866.3
700/166
|
References Cited
U.S. Patent Documents
4125755 | Nov., 1978 | Plamquist | 219/121.
|
4462046 | Jul., 1984 | Spight | 358/101.
|
4578554 | Mar., 1986 | Coulter | 219/121.
|
4765532 | Aug., 1988 | Uomoti et al. | 228/212.
|
4814576 | Mar., 1989 | Morita et al. | 219/121.
|
4872940 | Oct., 1989 | Strum et al. | 156/379.
|
4973817 | Nov., 1990 | Kanno et al. | 219/121.
|
5030313 | Jul., 1991 | Takeda et al. | 156/380.
|
5064991 | Nov., 1991 | Alborante | 319/121.
|
5115115 | May., 1992 | Alborante | 219/121.
|
5142118 | Aug., 1992 | Schlatter | 219/121.
|
5169051 | Dec., 1992 | Noe | 228/5.
|
5190204 | Mar., 1993 | Jack et al. | 228/5.
|
5229571 | Jul., 1993 | Neiheisel | 219/121.
|
5272312 | Dec., 1993 | Jurca | 219/121.
|
5325443 | Jun., 1994 | Beatty et al. | 382/8.
|
5328083 | Jul., 1994 | Peru et al. | 228/5.
|
5451742 | Sep., 1995 | Nishio et al. | 219/121.
|
5502292 | Mar., 1996 | Pernicka et al. | 219/121.
|
5510597 | Apr., 1996 | Kampmann et al. | 219/137.
|
5533146 | Jul., 1996 | Iwai | 382/150.
|
5580636 | Dec., 1996 | Kampmann et al. | 428/119.
|
5586139 | Dec., 1996 | Takenaka et al. | 372/99.
|
5591360 | Jan., 1997 | Mombo-Caristan | 219/121.
|
5595670 | Jan., 1997 | Mombo-Caristan | 219/121.
|
5607605 | Mar., 1997 | Musasa et al. | 219/121.
|
5616261 | Apr., 1997 | Forrest | 219/121.
|
5624585 | Apr., 1997 | Haruta et al. | 219/121.
|
5665255 | Sep., 1997 | Busuttil | 219/121.
|
5681490 | Oct., 1997 | Chang | 219/121.
|
5724712 | Mar., 1998 | Bishop | 29/430.
|
5728992 | Mar., 1998 | Swidwa | 219/121.
|
5742504 | Apr., 1998 | Meyer et al. | 364/188.
|
5760365 | Jun., 1998 | Milewski et al. | 219/121.
|
Foreign Patent Documents |
59-174289 | Oct., 1984 | JP | 219/121.
|
62-118994 | May., 1987 | JP | 219/121.
|
1058170 | Mar., 1998 | JP | 219/121.
|
Other References
Mombo-Caristan, Koch, and Prange, "Seam Geometry Monitoring for Tailored
Welded Blanks", ICALEO, pp. 123-132, 1991.
|
Primary Examiner: Heinrich; Samuel M.
Attorney, Agent or Firm: McDonald; Alan T., Ciamacco; Vince
Standley & Gilcrest LLP
Claims
What is claimed is:
1. A method of inspecting a laser weld between metal sheets of dissimilar
thickness, the method comprising:
capturing a two dimensional image of a laser weld bead;
measuring at least one characteristic of the laser weld bead image; and
comparing the value of the at least one characteristic of the laser weld
bead image with a reference value to determine the quality of the laser
weld.
2. The method of claim 1 wherein the at least one characteristic of the
laser weld bead is selected from the group consisting of bead width, top
mismatch, top concavity, top convexity, root width, bottom mismatch,
bottom concavity, and bottom convexity.
3. The method of claim 1 wherein the capturing of the image is accomplished
by a CCD camera.
4. The method of claim 1 further comprising:
generating a signal based upon the quality of the laser weld; and
outputting the signal to identify the quality of the welded work piece.
5. The method of claim 4 additionally comprising the steps of:
removing the welded work piece from the laser welder; and
depositing the work piece in a selected location based upon the quality of
the laser weld.
6. A method of inspecting a laser weld, the method comprising:
capturing a profile image of a laser weld bead;
measuring at least one characteristic of the laser weld bead image; and
comparing the measured value of the at least one characteristic of the
laser weld bead image to a reference value to determine the quality of the
laser weld.
7. The method of claim 6 wherein the at least one characteristic of the
laser weld bead image is selected from the group consisting of bead width,
top mismatch, top concavity, top convexity, root width, bottom mismatch,
bottom concavity, and bottom convexity.
8. The method of claim 6 wherein the capturing of the image is accomplished
by a CCD camera.
9. The method of claim 6 further comprising:
generating a signal based upon the quality of the laser weld; and
outputting the signal to identify the quality of the welded work piece.
10. The method of claim 9 further comprising:
removing the welded work piece from the laser welder; and
depositing the work piece in a selected location based upon the quality of
the laser weld.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a new automated laser welding
system configured to produce, for example, an improved welded work piece,
such as an automotive body panel, and a system and method for the
manufacture thereof that includes an improved laser welder and a visual
weld inspection device. The invention also relates to a method for
performing an automated quality control inspection of a laser weld.
2. Background
In the past, welded work pieces such as body panels for use in the
automotive vehicle industry were made by stamping or drawing the panel
from either a single blank of a ductile sheet metal material, including
steel, or from a plurality of such blanks that were previously welded
together. Either type of welded work piece or body panel usually required
the addition of stiffeners and pads welded to sections of the panel to
improve its structural rigidity. The added stiffeners and pads were also
needed to increase the thickness of the work piece in predetermined
locations so that various structural and fastening assemblies could be
fastened and welded to the panel without damage during the fastening or
welding process. The addition of the stiffeners and pads increased the
weight of the work piece and also increased the total manufacturing time
needed to fabricate the work piece. The work pieces were often formed,
drawn, or stamped into a final shape to have a three-dimensional shape
corresponding to the overall design of the automotive vehicle.
As a result of the number of manufacturers in the field, the automotive
vehicle industry is very competitive with respect to, among other things,
quality, raw material costs, and manufacturing times required to
completely fabricate and assemble a vehicle. To remain competitive,
manufacturers have continuously expended enormous resources to contain, if
not reduce, material costs by reducing part weight, part count, and
manufacturing time while maintaining the needed high degree of quality. A
considerable amount of such resources have been directed to improving and
automating routine tasks such as the fastening together of various work
pieces and vehicle parts such as, for example, body panels for fenders,
quarter panels, trunk lids, engine compartment hoods, vehicle doors, and
other various components.
Previously, multi-part sheet metal blanks have been welded together into a
single work piece before being stamped into a final shape. These blanks
were prepared by a variety of fastening techniques including chemical,
arc, and CO.sub.2 laser welding, riveting, bolting, cold forming, and
similar methods. Of particular interest in recent years is the use of more
efficient laser welding using CO.sub.2 lasers in automated, numerically
controlled manufacturing processes. Such laser welding can be accomplished
for joining together sheet metal blanks at a common seam by means of, for
example, a lap weld, or a butt weld. Butt welds are often preferred
because only a single seam needs to be welded in contrast to lap joint
which usually require that two seams be welded.
Many problems have been associated with the use of CO.sub.2 lasers
including the requirement that less than optimum welding speeds must be
used because of the poor absorption by steel work pieces of the energy
produced by the CO.sub.2 laser. Also, laser welded joints can be plagued
with problems despite the use of an appropriate weld speed if a
manufacturer does not carefully prepare the work pieces or is otherwise
not attentive to the intricacies and pitfalls of laser welding processes.
Problems are even more prevalent when the blanks to be welded together are
of dissimilar thickness. Such problems include, for example, mismatch
between the welded parts along the joint on at least one exterior surface,
poor weld bead dimensions or hardness, cracks, poor weld bead continuity
across the length of the weld, and pinholes formed in the weld bead. Many
of these welding problems are difficult to avoid and even more difficult
to detect. More often than not, detection of such problems can only be
accomplished by a slow and tedious visual inspection. Further, some of
these problems, such as cracks, weld spatter, and pinholes, can only be
detected through destructive testing such as by tension and shear tests,
micrographic cross-sectional analysis, etch and penetrant dye inspections,
and formability testing to ensure the welded blanks of the work piece can
be drawn or stamped without failure anywhere along the welded joint.
These problems are especially apparent when steel work pieces, such as
welded components for an automotive body or door panel, are to be butt
welded together for form a larger, single work piece or door panel blank
that can be later stamped or drawn into a shaped panel ready for painting
and attachment to the vehicle. In many cases such welds are straight line
weldments that could be completed faster if an improved laser welding
technique were available. Additionally, it would be desirable to have an
automated manufacturing assembly line wherein multiple work pieces could
be automatically introduced to the laser welding apparatus to minimize the
risk of injuries to workers from reflected laser energy. Further, such
welding manufacturing processes could be made more efficient if a
technique existed to speed up the post-weld inspection process.
There have been attempts to develop a viable method for laser welding
inspection. U.S. Pat. No. 5,607,605 discloses such a method, which
utilizes a CCD (Charge Coupled Device) camera to capture an image of the
plasma generated when a laser beam contacts an object to be welded. The
image is then sent to an image processing device, which measures a
selected particular feature of the plasma cloud. The measurement is
further transferred to a distinction device, which compares the
measurement with a reference value to determine if the laser welding
condition, and thus the weld, is acceptable.
Electro-optical detection of laser welding conditions has also been
employed as an inspection method. U.S. Pat. No. 5,272,312 recites a method
for the inspection of a laser weld, wherein the area of the material in
contact with the laser beam, referred to as the laser processing spot, is
projected onto at least one photodetector such as photodiode which detects
the amount of liquid material ejected from the weld pool during the
welding process. The signal from the photodiode can be converted into an
electrical signal, which may then be sent to a processing unit for
determination of the size and location of voids or pores in the weld seam.
In one embodiment, this reference discloses the detection of ultraviolet
radiation present in the plasma cloud.
Laser welding generates particular signals which may be monitored to
determine the quality of a weld. U.S. Pat. No. 5,681,490 discloses that
sensors such as photodiodes, phototransistors, photo darlingtons,
pyroelectric detectors, microphones, and infrared and thermal detectors
can be positioned to monitor various stages of the welding process. Such
sensors may be utilized to monitor light, sound, gas, smoke, temperature,
etc. The signals generated by these sensors may then be analyzed by a
computer to predict the weld quality.
None of the prior art, however, discloses an apparatus or method utilizing
direct inspection of the weld bead to determine the quality of a laser
weld. The prior art methods generally depend upon the use of unstable
process indicators to ascertain the condition of the weld, often requiring
the monitoring and analysis of a multitude of signals to reach a
conclusion regarding weld quality.
The automotive industry is in need of a laser welded work piece that
contains fewer parts, has an optimally minimized weight, and that is
produced through the use of an automated, laser welding manufacturing
process. The welded work piece produced in accordance with the present
invention, and the system and method for its manufacture, overcomes the
deficiencies of the presently known methods for automated laser welding
and inspection of welded work pieces.
SUMMARY OF THE INVENTION
In general, the present invention is directed to an improved laser welded
work piece and an automated laser welding and visual inspection system and
method configured to manufacture the work piece. The welded work piece
incorporates a minimized gap that is designed to improve the structural
properties of the laser weld. The new automated manufacturing system
includes a robotically automated production line configured to prepare
blank work pieces for welding by precision shearing at least one edge, and
to precisely align the blanks and laser weld them together using a single
or dual cell, high-speed, high-power laser. During welding, the laser weld
is concurrently inspected by a visual inspection device to determine
whether the welded work piece should be accepted or rejected. The operator
can continuously supply palletized raw materials, such as pallets or skids
of sheet metal blanks, to the production line without stopping or
interrupting the automated production line. After welding, the system
robotically sorts and re-palletizes the finished, welded work piece onto
accepted work piece skids or onto rejected work piece skids. The operator
can remove the accepted and rejected work pieces from the production line
without stopping or interrupting the continuously running line.
THE WELDED WORK PIECE
The invention includes a welded work piece for use in manufacturing an
automotive vehicle that incorporates a first blank of a steel sheet stock
with a first thickness and having at least one first precision sheared
edge, and a second blank formed from a steel sheet stock material of a
second thickness having at least one second precision sheared edge. The
first and second precision sheared edges are produced in the respective
first and second blanks to form a minimized gap between the edges before
welding. The edges are laser welded using the apparatus disclosed herein,
to form a beaded seam that permanently joins the respective first and
second blanks. In a method for manufacturing the improved welded work
piece, first and second blanks of a sheet stock steel are selected to be
of similar or dissimilar respective thickness and respective precision
sheared edges. The edges are positioned on a flat welding surface and
tightly compressed together in an abutting relationship to form a
minimized gap between the edges. The edges are then laser welded together
to form a beaded seam that permanently joins the blanks together to form
the welded work piece.
There is thus disclosed a welded work piece for use in manufacturing an
automotive vehicle, comprising first and second sheet metal blanks, each
formed with at least one precision sheared edge and having similar or
dissimilar thickness. The blanks form a minimized gap when the respective
at least one precision sheared edges are positioned in an abutting
relationship A continuous wave laser butt welded seam fixedly joins the
blanks together along at least one of the respective precision sheared
edges.
THE SYSTEM
Another aspect of the present invention is directed to a system for
manufacturing a welded work piece. The system includes at least one
articulating arm feeder robot, configured to retrieve at least one sheet
metal blank from a plurality of such blanks, from at least one of a
plurality of feeder skids containing palletized sheet metal blanks. The
arm is adapted to transport the individual blanks, one at a time, from the
skid to a load position on a magnetic conveyor. Each of the blanks are
formed with at least one joining edge. In applications where blanks of
dissimilar thickness or other dimensions are used, the blanks may either
be stacked alternately on a single skid, or a second articulating arm
feeder robot may be employed to retrieve a dissimilar blank from a second
plurality of such blanks that are palletized on a second plurality of
skids. The second robot arm operates cooperatively to feed the second,
dissimilar blank onto the magnetic conveyer.
The magnetic conveyor of the system is adapted to receive from the feeder
robot or robots at least two blanks. The conveyer is configured with blank
locator devices adapted to precisely position them on the substantially
flat conveyor bed. The blanks are proximally pre-positioned so each of the
respective joining edges are substantially parallel. The magnetic conveyor
is further configured to releasably restrain the positioned blanks into
place and to move the blanks from the load position to a shear position.
The system also incorporates a precision shear device positioned about the
shearing position of the magnetic conveyer. The shear device is configured
with at least one upper stamping die that cooperates with at least one
lower stamping platen to precisely shear at least one of the respective
joining edges of each of the blanks. After shearing, the blanks are moved
by the magnetic conveyer onto an idle station that temporarily stores the
sheared blanks until they can be welded. The blanks are then conveyed by a
second conveyer to a welding gantry located at the other end of the idle
station.
The second conveyer moves the sheared blanks onto a laser weld bed of the
welding gantry. The gantry includes a clamping and positioning assembly
operative to releasably register and press the respective joining edges of
the blanks flat against the weld bed and tightly together with the edges
in an abutting relationship to form a minimized gap. The clamping
mechanism is configured with a clamp assembly having multiple bars that
clamp down on each blank to firmly press them against the laser weld bed.
The positioning assembly includes a plurality of locator assemblies that
push against one or more of the non-joining edges of each blank to
precisely locate the blanks so that the precision sheared edges are
tightly pressed together. When so pressed together, the edges form a
minimized gap or seam therebetween.
The system also includes a laser welder movably attached to the welding
gantry. The laser welder may be configured with a weld head powered by a
remote laser power unit. The weld head moves along the gantry and, when
energized, projects a laser beam incident to and focused upon the gap or
seam of the blanks to form a weld bead seam. The system also comprises a
laser weld inspection device that is adapted to move along either in
conjunction with or independently of the laser weld head to inspect to the
weld bead seam. Once welded, an exit conveyor operates to remove the
welded work piece from the laser weld bed. An articulating arm exit robot
is also included that is configured to move the work piece from the exit
conveyor to an exit station. If the inspection revealed that the weld was
acceptable, the exit robot moves the welded work piece to one of a
plurality of skids for work pieces that have passed the inspection.
Otherwise, if the inspection revealed that the weld bead seam was not
acceptable, the exit robot moves the defective welded work piece to one of
a possible plurality of reject skids.
There is thus disclosed a system for manufacturing a welded work piece,
comprising at least one articulating arm feeder robot, configured to
retrieve at least one blank from at least one of a plurality of feeder
skids of palletized sheet metal blanks. Each blank is formed with at least
one joining edge, and the articulating arm feeder robot is adapted to
transport the blank to a load position on a magnetic conveyor. The
magnetic conveyor is adapted to receive from the feeder robot or robots at
least two of the plurality of blanks and to precisely position them on a
conveyor bed. The blanks are proximally pre-positioned so each of the
respective joining edges are substantially parallel, and the magnetic
conveyor is further configured to releasably restrain the positioned
blanks into place and to move them from the load position to a shear
position. The system further comprises a precision shear device,
positioned about the shearing position of the magnetic conveyer, and
configured with at least one upper stamping die that cooperates with at
least one lower stamping platen to precisely shear at least one of the
respective joining edges. There is also a welding gantry, spaced apart
from the precision shear device and configured with a second conveyor
having a laser weld bed and connected to the magnetic conveyer with an
idle station therebetween. The second conveyer is configured to slidably
receive the sheared blanks from the idle station and to move them onto the
laser weld bed. The system also utilizes a clamping and positioning
assembly operative to releasably register and press the respective joining
edges of the blanks flat against the weld bed and tightly together in an
abutting relationship to form a minimized gap. A laser welder is movably
attached to the welding gantry, and has a weld head powered by a remote
laser power unit to project a laser beam incident to and focused upon the
gap for welding the blanks along the gap to form a weld bead seam. A laser
weld inspection device is slidably coupled to the welding gantry and
operative to inspect the weld bead. After inspection, an exit conveyor
coupled to the second conveyor, removes the welded work piece from the
laser weld bed. An articulating arm exit robot moves the work piece from
the exit conveyor to an exit station, which is selected from the group of
one of a plurality of accepted work piece skids or a rejected work piece
skid.
The system further includes a light curtain system that is configured to
surround each of the plurality of the feeder and exit station skids to
allow removal and replacement of empty feeder and full exit skids without
the need to interrupt the operating manufacturing system. If an operator
approaches any of the skids for removal and replacement, the light
curtains signal the robots either directly or indirectly. In response,
each of the robots is directed to another of the plurality of skids for
purposes of retrieving unwelded blanks or outputting welded work pieces
during the period of time that the light curtain is activated. Similarly,
each of the skids or a skid holder unit incorporates a sensor that either
signals that the skid is empty or full. If either of these conditions
occurs, the robot is directed to use another of the plurality of skids.
There is further disclosed a method for manufacturing a welded work piece
comprising the steps of: shearing a precision edge on a respective joining
edge of a plurality of sheet metal blanks, using a precision shear device
configured with at least one upper stamping die that cooperates with at
least one lower stamping platen to perform the shearing operation; moving
the plurality of precision sheared blanks together on a conveyor from the
precision shear device to a laser weld bed of a welding gantry; precisely
locating the blanks to register the precision sheared edges in a
compressed, abutting relationship; clamping the respective joining edges
of the blanks flat against the weld bed and tightly together in an
abutting relationship to form a minimized gap; and laser welding the edges
to form a beaded seam and to permanently join the blanks together.
THE LASER WELDER
In yet another aspect of the present invention, a single or multi-celled
laser welder is described. The laser welder incorporates at least one
laser weld head that is configured to movably project at least one laser
beam onto a plurality of work pieces to weld them together. As described
above, the work pieces are positioned so that the edges are tightly
pressed together in an abutting relationship to form a seam or gap. The
work pieces are welded together with a laser weld head that projects the
laser beam incident to the gap with a compound angle. The compound angle
is measured relative to the vertical direction substantially normal to the
substantially flat sheet metal work pieces. A leading angle component of
the compound angle is substantially in the direction of movement of the
laser weld beam as the weld head moves across the blanks during welding. A
leaning component of the compound angle is orthogonal to the leading angle
and is substantially in the direction normal to the blanks and the gap and
it leans to one side towards one of the blanks away from the vertical
direction.
Thus, there is disclosed a laser welder for welding a plurality of work
pieces, comprising a laser weld head configured to movably project a laser
beam onto a minimized gap formed between a plurality of adjacent,
substantially flat work pieces formed with respective precision sheared
edges. The edges are positioned in an abutting relationship, and the laser
weld head is operative to weld the edges by forming a weld bead seam
between the edges. The laser welder further comprises a laser beam
incident on the gap with a compound angle. The compound angle is measured
relative to the vertical direction substantially normal to the work
pieces, and includes a leading angle component substantially in the
direction of movement of the laser weld beam, and a leaning component
substantially in the direction normal to the gap and leaning towards one
of the blanks away from the vertical direction.
There is further disclosed a multi-celled laser welder comprising a
plurality of laser weld heads, each configured to movably project a laser
beam onto a plurality of minimized gaps formed between a plurality of
adjacent, substantially flat work pieces formed with respective precision
sheared edges. The edges are positioned in an abutting relationship and
the laser weld heads are operative to weld the edges by forming a weld
bead seam between the edges. The laser beams are incident on the gaps with
a compound angle. The compound angle is measured relative to the vertical
direction substantially normal to the work pieces, and includes a leading
angle component substantially in the direction of movement of the laser
beams and a leaning component substantially in the direction normal to the
gaps and leaning towards one of the blanks away from the vertical
direction.
THE INSPECTION SYSTEM
The present invention is also directed to a specially designed vision
system configured to inspect a laser weld bead in real time. When the
focal point of a laser beam contacts a work piece, it generates intense
heat which forms a molten weld pool. As the laser beam traverses the work
piece, the weld pool left behind quickly cools to form a weld bead. A
visual sensor, such as a CCD (Charge Coupled Device) or video camera
follows the laser welding head to view the weld bead. Although in a
preferred embodiment of the invention a visual sensor is affixed to and
travels with the laser welding head, it should be realized that the visual
sensor could also be detached and independently propelled. An image of the
weld bead is captured by the visual sensor at a predetermined interval
based on the velocity of the laser welding head and other factors. The
image from the visual sensor is sent to an image processing board, which
in conjunction with a coprocessor board, computer and the system software,
compare the image to a list of predefined, preferred tolerances, which
correlate with several established characteristics of the weld bead
considered to be acceptable. If it is determined that the selected
characteristics of the weld bead image are within the specified predefined
tolerance limits, a signal is generated classifying the weld as
acceptable. If it is determined that the image is outside the specified
predefined tolerance limits, a signal is generated classifying the weld as
defective.
Thus, there is disclosed a laser welding inspection system comprising a
laser welding device, an image capturing device for capturing the image of
a laser weld bead, and an image processing device in electronic
communication with the image capturing device, for measuring at least one
dimension of the laser weld bead image captured by the image capturing
device. The system further comprises a distinction device, in electronic
communication with the image processing device, for comparing the value of
the at least one dimension of the laser weld bead image measured by the
image processing device with a reference value, to determine the quality
of the laser weld.
There is further disclosed a method of inspecting a laser weld, the method
comprising capturing an image of a weld bead, measuring at least one
dimension of the laser weld bead image, and comparing the value of the
dimension of the laser weld bead image with a reference value to determine
the quality of the laser weld.
Other features and advantages of the invention will become apparent from
the following detailed description, taken in conjunction with the
accompanying drawings, which illustrate, by way of example, the features
of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Without limiting the scope of the present invention as claimed below and
referring now to the drawings, wherein like reference numerals across the
various views refer to identical, corresponding, or equivalent features
and parts:
FIG. 1 is a platform view, in reduced scale, of a preferred embodiment of a
welded work piece produced in accordance with the present invention;
FIG. 2 depicts a rotated, partial, cross-sectional view taken along section
line 2--2 of FIG. 1, in enlarged scale, of the work piece of FIG. 1 before
welding;
FIG. 2a depicts a dimensional representation of an illustrative example of
the work piece of FIG. 2;
FIG. 3 is a rotated, partial, cross-sectional view taken along section line
3--3 of FIG. 1, in enlarged scale, of the work piece of FIG. 1 after
welding;
FIG. 3a is a schematic representation of another embodiment of the work
piece of FIG. 3;
FIG. 3b is a schematic representation of another embodiment of the work
piece of FIG. 3 wherein the lower surfaces of the welded work piece are
misaligned;
FIG. 4 is an elevated perspective view, in reduced scale, of a stamped body
panel fabricated from the welded work piece of FIG. 1;
FIG. 5 is schematic top-view, in reduced scale, of the layout of a system
for manufacturing the laser welded work piece of the present invention;
FIG. 6a is a schematic, rotated front-view taken along section line 6--6 of
FIG. 5, in enlarged scale, of a representative laser welder gantry, a
laser welder, and a laser weld inspection device;
FIG. 6b is a detail view, in enlarged scale, of a portion of FIG. 6a and
showing the leading angle component of the compound angle of the laser
beam;
FIG. 6c is a section view taken along line 6c --6c of FIG. 6b and showing
the leaning angle component of the compound angle of the laser beam;
FIG. 7 is a flow diagram representative of an exemplary embodiment of the
comparison procedure used by the control computers or the laser weld
inspection device, or both, of FIGS. 5, 6a, and 6b;
FIG. 8 is an enlarged view of a portion of FIG. 5 representing a plurality
of the feeder skids, feeder robot arm, and light curtains of the
manufacturing system of the present invention;
FIG. 9 is an enlarged view of a portion of FIG. 5 representing a plurality
of the feeder skids, feeder robot arm, part of the magnetic conveyor, and
part of the precision shear machine of the manufacturing system of the
present invention;
FIG. 10 is an enlarged view of a portion of FIG. 5 representing a second
conveyer, a weld gantry, a laser weld head, and work piece locators of the
manufacturing system of the present invention; and
FIG. 11 is an enlarged view of a portion of FIG. 5 representing an exit
conveyer, an exit robot arm, and a plurality of accepted work piece skids
and a rejected work piece skid of the manufacturing system of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention produces an improved welded work piece that is welded from a
plurality of sheet metal blanks of substantially similar or dissimilar
thickness, which are formed from materials such as steel, aluminum, and
alloys. According to the invention, the plurality of blanks are laser
welded together at speeds faster than previously possible with a much
lower percentage of rejected work pieces. Before welding, the blanks are
precision sheared along at least one joining edge. The precision shear
ensures that when the sheared edges of the blanks are placed in an
abutting relationship, a minimized gap, if any at all, will exist. The
closely toleranced and minimized gap improves the final weldment and
reduces the time needed to complete the manufacturing process.
With reference to FIG. 1, an improved work piece 10 is shown having a first
sheet metal blank 20 and a second sheet metal blank 30. The blanks 20, 30
each have at least one joining edge 22, 32 each formed with a precision
sheared edge 25, 35, respectively. Prior to butt welding, the blanks 20,
30 are pressed flat onto a welding surface (not shown) with the precision
sheared edges 25, 35, pressed tightly together. As can be understood with
reference to FIGS. 2 and 2a, when the edges 25, 35 are so positioned, they
are ideally in facing contact with one another over the entire length of
the interface between them. However, under even the most controlled and
the most tightly toleranced of manufacturing conditions, uninterrupted
contact of the edges 25, 35 is unachievable, and some gaps 38, although
very small, are experienced at the interface of the edges 25, 35. These
gaps 38 are due to the normal manufacturing tolerances encountered with
the manufacturing process and are always experienced during production. In
the preferred embodiment, the gaps 38 between precision sheared edges 25,
35 are preferably between approximately zero and approximately 0.08
millimeters and more preferably less than 0.04 millimeters.
These gaps 38 can cause problems with welded joints such as butt welds.
This is because the gaps 38 can lead to less than optimum welds due to the
development of macroscopic and microscopic cracks and micropores, among
other irregularities and anomalies, in the weldment between the joining
edges 22, 32. If not properly accounted for in the design, strength
analysis, and manufacturing process, or if not otherwise minimized during
the manufacturing process, these types of irregularities and anomalies can
lead to increased numbers of rejected welded work pieces 10. Despite such
anomalies and irregularities, the gaps 38 can be significantly minimized
during the manufacturing process which, in turn, substantially reduces the
number of rejected work pieces 10. The gaps 38 are minimized by precision
shearing the joining edges 22, 32, to achieve the above gap dimensions,
before welding. FIGS. 3, 3a, and 3b schematically represent the welded
interface or weldment 40. The preferred characteristics of the weldment
are described below with respect to the discussion of the laser weld
inspection device of the present invention.
The preferred embodiment of a welded work piece produced in accordance with
the present invention includes blanks 20, 30 of substantially similar or
dissimilar thickness. For purposes of illustration but not limitation, the
thickness for each blank 20, 30 can range preferably from between
approximately 0.4 millimeters to approximately 2.0 millimeters, and more
preferably from between approximately 0.7 millimeters to approximately 1.4
millimeters. In one embodiment of the invention, one blank thickness is
between approximately 0.50 millimeters and approximately 0.75 and the
other blank thickness is between approximately 1.25 millimeters and
approximately 1.50 millimeters. As an example of such dissimilar
thickness, the first blank 20 can be selected to have a thickness of
approximately 0.7 millimeters while the second blank 30 can be selected to
have a thickness of approximately 1.4 millimeters. Many various thickness
arrangements of first and second blanks 20, 30 are possible. The preceding
example is particularly effective for use in manufacturing a welded work
piece suitable for applications where increased rigidity must be imparted
to a portion of the work piece. This example is especially efficient in
applications where the added rigidity must be accomplished without a
corresponding increase in the part count or the part weight, as would
occur if stiffeners, pads, or other structural supports were added to the
work piece.
Although representative dimensions are set forth, they are presented only
for purposes of demonstrating a particular embodiment of the present
invention and not for purposes of limitation. One having ordinary skill in
the art will understand that various types and thicknesses of steel, steel
alloy, and other metal materials, are contemplated for use with the
present invention.
As a further illustrative example, the usefulness of the preferred
embodiment is readily apparent in aeronautical or automotive applications.
In aeronautical applications, structural rigidity must be accomplished
with minimum possible weight and part count to minimize the overall
aircraft weight and manufacturing cost. Similarly, in automotive
manufacturing, where millions of copies of the same part are fabricated,
minimized weight and part count can translate into substantial savings in
material costs and manufacturing times. With reference to FIG. 4, a
representative preferred embodiment of the improved welded work piece is
shown as applied to the fabrication of a drawn or stamped body panel 50
configured for use in the automotive vehicle manufacturing industry. First
and second blanks 20', 30' are welded together along weld seam 40' after
precision shearing the respective joining edges. The improved welded work
piece is then stamped or drawn into the desired shape of an automotive
vehicle body panel.
SYSTEM DISCLOSURE
The present invention also provides a significant improvement over
previously known welded work pieces and the system and method for their
manufacture. With reference to FIG. 5, the invention includes an automated
welded work piece manufacturing system 100 configured to robotically
retrieve a plurality of blanks, to form at least one respective precision
sheared edge on the blanks, to weld the blanks together, to inspect the
weldment, and to robotically output the satisfactorily welded work pieces
to an accepted station and the improperly welded work pieces to a rejected
station.
The blanks to be welded are fed to the automated manufacturing system 100
by at least one of plurality of articulating arm feeder robots. The
automated manufacturing system 100 includes one or more control computers
105 configured to communicate with, monitor, and/or control the various
subsystems and components of the system 100. With reference to FIGS. 5 and
9, also included is at least one of a plurality of robotic stations that
preferably include at least a first robotic feeder station 110 that
incorporates a first articulating arm feeder robot 120 that is capable of
variable operation speeds and of 1, 2, or 3 dimensional motion with
articulation about substantially 2 to substantially 5 axes each ranging
between approximately 5 to approximately 360 degrees. Although not
required for purposes of the preferred embodiment, each robot may also
incorporate a vertical and horizontal telescoping capability for added
flexibility. The first robot 120 further includes a manipulator 125
configured to releasably capture a first sheet metal blank 130 of a
plurality of blanks 135 for transport between at least two positions. Many
types of suitable robots are commercially available and include the Model
SK-120 industrial robot available from Motoman, Inc., of West Carrolton,
Ohio.
Preferably, the first robot 120 retrieves the blank 130 from at least one
of a first plurality of feeder pallets or skids 140 containing the
plurality of palletized blanks 135. Typically, the first robot is
configured to empty one skid at a time and then to begin removing blanks
from the next available skid. The first robot 120 can be configured to
retrieve a predetermined number of blanks 130 from the skid 140 before it
begins to retrieve blanks 130 from the next available skid 140.
Alternatively, the skids 140 themselves may be configured to detect a low
quantity or empty skid. To accomplish this, each of the skids 140 includes
a quantity detector assembly 145 adapted to determine whether the quantity
of remaining blanks 130 is low or zero, or both, by using either a light
sensor, a weight detector, or a video system, or some combination thereof.
Upon determining a low or zero quantity, the detector assembly 145 signals
either the first robot 120 or any of the control computers 105. In
response, either the first robot 120 or any of the control computers 105
can take a selected skid 140 out of service and initiate retrieval of
blanks 130 from the next available skid 140. For purposes of alerting
supervisory personnel of the need to refill the empty skid 140, the
detector assembly 145 can be adapted to also generate a visual, audible,
or electronic alarm or signal. In a modification to the preferred
embodiment, the quantity detector assembly 145 can be adapted for
attachment to the manipulator 125 of the first robot 120 so that the
detector assembly 145 detects low or empty skids 140 when it attempts to
capture and retrieve the next blank from the skid.
For safety purposes, the first robotic feeder station 110 is substantially
surrounded by a safety fence or partition 150 configured with light
curtain assemblies or specially adapted doors, or both, to detect
intrusions into the work area of the first robotic feeder station 110. The
partition 150 incorporates a plurality of spaced apart light curtain
sensors 155 operative to signal an alarm when an intrusion occurs. These
types of sensors are commercially available from many vendors including
Scientific Technologies, Inc. of Fremont, Calif. The alarm is selected to
be either visual, acoustic, or electronic, or a combination thereof The
alarm, in turn, is configured to generate one or more resulting alerts.
First, the alarm can visually or audibly warn an individual intruder or
supervisory personnel of the presence of an intrusion into the potentially
dangerous area adjacent to the first robot 120. Also, the alarm can
electronically signal the first robot 120 or any of the control computers
105, or both, that an intrusion of the first robotic feeder station 110
has occurred. If an intruder has entered the first station 110 work area,
then either the first robot 120 or one of the control computers 105, or
both, can generate further alarms. The first robot 120 can also be
configured to completely cease operation to prevent injury to the
intruder. Similarly, any of the control computers 105 can be configured to
stop the first robot 120 from operating, and can further stop the entire
system 100 from operations so the intruder can safely exit the first
station 110.
Additionally, the light curtain partition 150 includes spaced apart light
curtain skid removal sensors 160 proximate to each of the feeder skids 140
of palletized blanks 135. The removal sensors 160 are configured to
operate in cooperation with the quantity detector assemblies 145 and to
communicate a skid removal alert signal to either the robot 120 or any of
the control computers 105 when an operator approaches and engages any of
the feeder skids 140 for removal of the empty or low quantity skid 140 and
replacement with a full skid 140. When a skid 140 has already been
identified as empty or low, the removal sensor 160 can be adapted to
generate the skid removal signal or to remain silent. Also, even if the
detector assembly 145 has not generated a skid low or empty alert signal,
the skid removal sensor 160 is configured similarly to the detector
assembly 145 to alert the first robot 120 or any of the control computers
105 that blanks 130 must be retrieved from the next available skid 140.
Thus, an operator can easily remove and replace any skid 140 that is low
or empty. When a skid removal alert signal is generated, the first robot
120 is configured to retrieve blanks 130 only from one of the other skids
140 of the plurality. In this way, the system 100 can continue full speed
operations while empty or low skids are replaced with full skids.
As depicted in FIGS. 5 and 9, the system 100 may include a second robot
feeder station 170 configured with some or all of the capabilities of the
first robotic feeder station 110 including, for example, a second robot
175. For purposes of illustration but not limitation, the second station
170 is spaced adjacent to or apart from the first station 110. It can
include, for example, each or combinations of the components, assemblies,
and capabilities of the first station 110. The second station 170 is
configured to retrieve a second sheet metal blank 180 from a plurality of
palletized blanks 185 that are stored on a plurality of feeder skids 190.
As before, the skids 190 have some or all of the elements of the feeder
skids 140. The blanks 180 may have a thickness similar or dissimilar to
the blanks 130.
The one or more robotic stations 120, 175 are located proximal to a
precision conveyer assembly 200 that incorporates a precision shearing
machine 210, as can be understood with reference to FIGS. 5, 8, and 9. The
precision conveyor assembly 200 may be configured with a substantially
flat bed or a pre-shaped support jig 215 and plurality of precision
sub-conveyors 220, 225 configured to precisely locate relative to one
another, a plurality of work piece blanks such as blanks 130, 180. The
precision conveyor 200 is adapted to releasably restrain the received
blanks 130, 180 after they are received from the robotic feeder stations
110, 170. The restraint mechanism may include a plurality or combination
of vacuum, magnetic, or clamping devices arranged on a conveyor bed 230
about the conveyer assembly 200 or sub-conveyers 220, 225 to releasably
capture the blanks 130, 180. After receipt and capture of the blanks, the
conveyer assembly 200 or sub-conveyers 220, 225 precisely position the
blanks 130, 180 relative to each other and move them into a machining or
shearing position proximal to the precision shearing machine 210. The
conveyor assembly 200 or sub-conveyers 220, 225 each include actuatable
locator assemblies 228 arranged about the conveyer bed 215 that are
operative to engage or push against one or more of the exterior sides of
the blanks 130, 180 to orient and position the blanks relative to each
other and the shearing machine 210 with the joining edges of the blanks
130, 180 arranged in a facing and substantially parallel relationship. The
locator assemblies 228 are preferably actuated with air-cylinders or any
or a wide variety commercially available industrial air, pneudraulic, and
hydraulic systems. Many suitable conveyors having such positioning
assemblies are available from a number of commercial suppliers and a
magnetic conveyor suitable for use as the above configuration is available
from VIL Magnetic Conveyors of Chicago, Ill.
The preferred precision shearing machine 210 includes dual shear dies or
blades and corresponding platens (not shown) configured to simultaneously
shear a portion of a joining edge from each of the blanks 130, 180. Each
of the dies and corresponding platens incorporate precision machined
outside corner edges that are precisely aligned with each other to impart
a precision sheared edge on each of the blanks 130, 180. Each die and each
platen can also be fabricated to include a plurality of precision machined
edges so that the dies and platens may be removed, reversed or rotated,
and replaced when one of the edges becomes worn or out of tolerance. In
this way, each die and platen may be reused more than once before the
outside corner edges of each die and platen must be remachined to restore
the precision toleranced edge. Accordingly, each die and each platen can
preferably include four precision machined edges. Alternatively, at least
two precision machined edges are achievable. Each edge is machined to
impart a precision sheared edge to each blank 130, 180 so that when
positioned into an abutting relationship, the sheared edges are
substantially in uniform contact with each other with a minimized gap
therebetween of between approximately zero and 0.08 millimeters.
Preferably, between approximately 1 millimeter and approximately 10
millimeters are removed from the blanks. More preferably, between
approximately 3 millimeters and approximately 5 millimeters are removed.
Removal of this amount of material assures that enough material is removed
to eliminate possible edge defects in the raw stock material. Also,
removal of at least this amount of material ensures that a clean shear
results with a minimized amount of possible tolerance anomalies. In turn,
when the precision sheared edges are pressed together, the interface
between the edges will be in substantially uniform contact with a
correspondingly minimized gap therebetween. The use of dual shear dies and
platens improves efficiency because shearing of both blanks 130, 180 is
accomplished in a single step. However, a single die or blade is a
suitable alternative for lower throughput applications. The preferred
precision shearing machine 210 also automatically removes the sheared
scrap from the machine before the blanks are transferred from the machine.
For purposes of illustration only, and not for limitation, a suitable
hydraulically operated, dual die, precision shearing machine is available
from VIL of Chicago, Ill.
After shearing the joining edges of the blanks, the conveyer assembly 200
or sub-conveyers 220, 225 release and transfer the blanks 130, 180 to an
idle or queuing station 230 as can be seen with reference to FIGS. 5 and
9. The queuing station 230 operates to temporarily store the sheared
blanks before welding. In an alternative embodiment, not shown, the idle
station 230 can be replaced with at least one transfer robot having some
or all of the capabilities of the first and second robots 120, 175. By
temporarily storing the sheared blanks 130, 180, another set of blanks may
be retrieved, aligned, and sheared by the precision shearing machine 210.
LASER WELDER DISCLOSURE
As illustrated in FIGS. 5 and 10, a second conveyor 240 slidably
repositions the blanks 130, 180 and moves them from the idle station 230
onto a welding bed 330 of a laser welding gantry 300. To protect workers
and other nearby equipment from injury due to reflected laser energy or
plasma, sputter, and other debris, the laser welding gantry 300 is
preferably enclosed. With reference also to FIGS. 5, 6a, and 6b, the
gantry 300 incorporates a numerically controlled laser welder 350
configured to move across the gantry 300 and incorporating a laser weld
head 355 and a laser weld inspection device 400. Although in some
applications a self-contained laser could be used, the preferred
embodiment of the present invention includes a laser weld head 355 that is
powered by a remote laser unit 380 through a fiber optic cable 360
contained in a cable support tray 390.
The laser welding gantry 300 incorporates an automated position adjustment
and orientation system having a plurality of pusher elements 305
retractably arranged on the second conveyer 240 about the laser weld bed
330. The pusher elements 305 are generally retracted down into the second
conveyer 240 until the blanks are moved onto the laser weld bed 330. Once
in place, the locator assemblies 305 are actuated and rise up to
releasably engage the exterior edges of the blanks 130, 180, pushing the
blanks into alignment so the precision sheared edges are registered
substantially in parallel with each other and compressed into tight
contact so that any gap between the precision sheared edges is minimized.
Such locator assemblies can be similar in design to those employed by the
precision conveyer 200. Once the edges are registered and in contact, a
clamping mechanism 310, spanning substantially across the width of the
gantry 300 above the welding bed 330, is deployed so a plurality of clamp
members 315 clamp down on the blanks 130, 180 to hold them in place
against the bed 330 during welding.
The laser welder 350 is movably attached to the welding gantry 300 and is
preferably numerically controlled by an appropriately programmed computer
that can include any of the control computers 105. The welder 350 is
controlled to maintain a precise speed as it is moved across the gantry
300 during welding. The welder includes a laser weld head 355 that is
connected by a fiber optic cable 360 to a remote laser power unit 380.
With reference to FIGS. 2, 3a, 3b, 6a, 6b, 6c, and 7, it can be understood
that the weld head 355 is configured to focus and project a laser beam 370
incident to and focused upon the minimized gap 38 between the precision
sheared edges blanks 130, 180 to irradiate the region around the precision
sheared edges to weld them together by forming a weld bead seam 40, 40'.
Many types of lasers are commercially available for various welding
applications. For purposes of the present invention, however, it is
preferable to use a single or dual cell (with corresponding single or dual
optical fibers), solid, non-pulsed, continuous laser such as a neodymium
doped, hard synthetic yttrium aluminum garnet laser (Nd-YAG). Preferably,
the laser has output power rating of at least approximately 2.5 to
approximately 3.0 kilowatts, and is preferably capable of generating a
power output at the laser weld head 355 of at least approximately 2.3 to
approximately 2.8 kilowatts, and more preferably a laser weld head 355
output of approximately 2.4 kilowatts. A suitable Nd-YAG laser includes
the Model LW-8 Laser Blank Welder available from Lumonics, Inc. of
Livonia, Mich. Similarly powered gas and pulsed lasers can be used
provided that they are capable of producing the specified power ranges.
Although a single cell, single fiber laser is represented by the figures,
a dual cell laser will be equally effective and will increase the
throughput of the laser welder 350 accordingly.
A gas jet 385 is also part of the laser welder and the gas stream is
directed in the forward direction following the direction of the travel of
the laser beam 370 and onto the region of the edges being irradiated by
the beam. The gas jet 385 produces a jet stream that reduces and ideally
eliminates gaseous contamination of the weld and to minimize plasma
shielding effects. The gas jet is preferably any of a number of inert
gases including, for example, argon, helium, or nitrogen, and can also be
directed against the underside of the weld region for additional
protection of the weld. The forward direction of the resultant jet stream
also "blows" the plasma cloud and other welding debris forward and away
from the laser weld head 355 and its associated and proximate components
including the inspection device discussed below.
With particular reference to FIGS. 6b and 6c, it can be seen that the laser
beam 370 is focused to intersect the seam or interface 40 of the blanks
and to irradiate their upper surface. The beam 370 projects at a compound
angle to the vertical direction perpendicular to the surface of the blanks
130, 180. The compound angle includes "leading" and "leaning" components.
The vertical direction is represented by the "Y" direction of the
reference coordinate system labeled "A" in FIGS. 6b and 6c. The "X"
direction represents the forward direction of the incident laser beam 370
across the gantry 300 during welding. With reference to FIG. 6b, the laser
beam 370 leading angle component is labeled ".theta." (theta) and it is
measured from the direction of the "Y" axis. Preferably, the leading
angle, .theta., is between approximately 5 degrees and 15 degrees, more
preferably between 7 and 12 degrees, and is most preferably approximately
10 degrees.
The leading angle serves several important functions. First, the leading
angle, .theta., prevents reflection of incident laser energy back into the
laser weld head 355 and, in turn, into the laser unit. Next, leading angle
.theta. allows the laser weld head 355 to lag the point on the surface
where welding occurs. This protects the weld head 355 from contacting the
plasma cloud and weld spatter and debris during welding. Third, leading
angle .theta. changes the shape of the laser beam spot that irradiates the
weld seam region. Ordinarily, the weld beam spot would be a circle if the
beam was perpendicular to the work piece surface. However, a circular weld
spot creates a very high energy density that creates welding problems that
are difficult to control by adjusting the speed of travel of the laser
beam 370. Thus, it has been found that by imparting an angle from the
perpendicular to the incident direction of the laser beam 370, the laser
beam spot will achieve an elliptical shape on the irradiated surface with
the major elliptical axis substantially parallel to the direction of
travel of the laser beam spot or the X direction of FIG. 6b. In turn, the
elliptical shape reduces the energy density on the irradiated surface by
spreading it over a larger area. The reduced effective energy density of
the laser beam spot allows better control of the welding process by
variance, for example, of a single welding parameter such as the speed of
travel of the laser beam spot across the weld seam. Such techniques are
described in a number of U.S. Patents including, for example, U.S. Pat.
No. 5,595,670 to Mumbo-Caristan which is hereby incorporated by reference
in its entirety.
With reference to FIG. 6c, the reference coordinate system A describes the
same Y direction as depicted in FIG. 6b. The "Z" direction points in the
lateral direction of the blanks 130, 180 (the Z direction is directed up
and out of the plane of FIG. 6b). In FIG. 6c, the X direction is directed
up and out of the plane of the view. The leaning angle component of the
compound laser beam angle is labeled ".gamma." (gamma). Preferably, angle
.gamma. is between approximately 1 and approximately 10 degrees, more
preferably between 3 and 7 degrees, and is most preferably approximately 5
degrees. The leaning angle .gamma. serves to further impart an elliptical
shape to the laser beam spot with the major elliptical axis due to angle
.gamma. substantially perpendicular to the direction of beam spot travel
and the X direction of FIG. 6b. When welding blanks of substantially
similar thickness, the beam spot is focused and positioned to irradiate
substantially equal regions of the blanks on both sides of the weld seam.
However, when welding blanks of substantially dissimilar thickness, the
leaning angle .gamma. is configured to precisely position approximately
between 15 percent and 30 percent, and more preferably approximately 25
percent of the cross-sectional area of the elliptical laser beam spot upon
the protruding vertical face of the thicker blank (see reference numeral
35 of FIG. 2). It will be understood that the remaining portion of the
beam spot will irradiate the thinner blank.
With these desired leading and leaning angles, the preferred speed of
travel of the laser beam spot across the blanks, as controlled by the
speed of the laser welder 350, that creates the optimum weld bead seam is
preferably between approximately 4 and approximately 10 meters per minute,
and more preferably approximately 7 meters per minute. These welding
parameters have been used to create a welded work piece wherein the welded
seam is at least 30 inches in length and has a tensile, pull strength
exceeding approximately 9,000 pounds.
These angles and speeds were empirically derived and are based upon
extensive trial and error experimentation because no data existed as to
how the Nd-YAG laser would perform in welding dissimilar thickness
materials at speeds greater than that possible with the prior art gas,
pulsed, and CO.sub.2 lasers. The preceding parameters have thus been
discovered to significantly minimize laser weld anomalies, such as
burn-through, cracking, and pores.
INSPECTION SYSTEM DISCLOSURE
The laser welding system 100 also preferably incorporates a laser weld
inspection and quality control device 400, as represented in FIGS. 6a and
6b, which is mounted to cooperate with the laser welder assembly 350 of
the gantry 300. In FIGS. 6a and 6b, the laser welding inspection device
400 is shown mounted to the gantry 300 to move along with the laser welder
350 during the welding operation.
As the laser weld head 355 projects the laser beam 370 to irradiate the
seam 40 between the work pieces 130, 180, a molten weld pool 402 is
generated at the focal point 404 of the laser beam 370. As the laser beam
370 and weld pool 402 traverse the seam 40, a weld bead is created. The
laser weld inspection device 400 utilizes an image capturing device,
namely a visual sensor 410, such as a CCD (Charge Coupled Device), or a
high shutter speed video camera. For purposes of illustration, and not
limitation, an example of a visual sensor 410 is the model MVS-5 camera
from Modular Vision Systems of Montreal, Canada. Preferably, the visual
sensor 410 is mounted rearward of the laser weld head 355 using a
structure such as a camera mounting bracket 420, although in an
alternative embodiment of the present invention, the visual sensor may be
self-propelled and have its own support structure. The visual sensor 410
is focused on the welding path, at a predetermined distance 425 behind the
laser's current focal point 404. The distance 425 between the focal point
of the laser 404 and the focal point of the visual sensor 430, is selected
so that the images captured by the visual sensor 410 will reflect a fully
solidified weld bead. In the preferred embodiment of the invention, the
distance 425 is between approximately 75 millimeters and approximately 200
millimeters, and more preferably between approximately 100 millimeters and
approximately 200 millimeters. Even more preferably, the distance 425 is
about 150 millimeters. The visual sensor 410 is also mounted at a specific
angle ".phi." (phi), preferably between approximately 5 and approximately
10 degrees toward the direction of travel, labeled as the X direction in
the figures. In the preferred embodiment of the invention, the visual
sensor 410 has a field of view of approximately 5 millimeters by
approximately 5 millimeters, although the field of view may be altered
based on the size of the weld to be inspected.
The visual sensor 410 is configured to capture images of the weld bead at
predetermined time intervals based on considerations such as the linear
velocity of the laser weld head 355 and the particular features of the
weld bead to be inspected. In one preferred embodiment of the present
invention, the visual sensor 410 captures approximately one image per
every 4 millimeters of travel while moving at a linear velocity of
approximately 6 meters per minute.
A representative cross-sectional view of a weld to be inspected is shown in
FIG. 3a. The blanks 130, 180 are selected to be of dissimilar thickness
and are fabricated from a material such as, for example, steel sheet metal
blanks. The blanks, are joined through a seam whose length is normal to
the surface of the paper. The blanks 130, 180 are joined by welding using
the laser welder 350. The intense heat of the laser beam 370 creates a
melt zone 440 as it contacts the seam 40 between the blanks. As this melt
zone cools, the weld bead 445, 450 forms on both the top and bottom
surfaces of the welded work piece.
FIG. 7 describes a representative comparison procedure that is included in
the system software of the present invention. As the laser weld head 355
follows the seam, the visual sensor 410 trails directly behind, viewing an
image of the fully formed weld bead 445 at predefined intervals. The
images captured by the visual sensor are electronically communicated to a
selected computer, such as one of the control computers 105, that
incorporates an image processor having image processing hardware or
software, or both. The image processor first analyzes the image to
determine the edges of the weld bead. The image processor then measures
the image in several preselected dimensional areas, which will be
described in detail below. The image is first measured for bead width A
and top mismatch B, (FIG. 3a). After bead width A and top mismatch B are
calculated, the image processor measures the image for top concavity C and
top convexity D. The selected computer, or any of the other control
computers 105, or both, also include a distinction device that
incorporates an image coprocessor having hardware or software components,
or both, and in electronic communication with the image processor. The
distinction device cooperates with the image processor to compare the
values of bead width A, top mismatch B, top concavity C, and top convexity
D with corresponding reference images or values, or both, that represent
the values of acceptable weld parameters and dimensions.
Once the comparison of each selected dimensional area is complete, the
distinction device determines the quality of the weld and whether the
welded work piece should be accepted or rejected. If it is determined that
the selected characteristics of the weld bead image are within the limits
of the predetermined, acceptable weld parameters and dimensions, a signal
is generated classifying the weld as acceptable. If it is determined that
the image is outside the predetermined, acceptable weld parameters and
dimensions, a signal is generated classifying the weld as rejected. The
signal may also be used to initiate the next appropriate machine process
step to be performed on the welded part. For example, the signal may be
sent to the removal station 510 described below and the articulating arm
robot 520 for removal to an accepted work piece skid 530 or a rejected
work piece skid 540.
As described in the procedure of FIG. 7, in a preferred embodiment of the
invention, the captured images and the reference images are analogized in
four specific dimensional areas Referring again to FIG. 3a, these
dimensional areas are depicted as bead width A, top mismatch B, top
concavity C, and top convexity D.
Bead width A is the distance between the two limit points defining the
edges of the weld bead 445, measured along an axis perpendicular to the
length of the seam on the top surface of the work pieces 130, 180.
As shown in FIG. 3a, the work pieces 130, 180 are preferably aligned to
have each of their bottom surfaces in the same plane. If the work pieces
are of substantially dissimilar thickness, then top mismatch B will exist
as the difference in height between the top surface of one blank 130, and
the top surface of the other blank 180. Although top mismatch B is
depicted in the preferred embodiment of FIG. 3a, the work pieces may also
be of substantially equivalent thickness, with both their top and bottom
surfaces lying in the same plane. In this case, mismatch between the
adjoining work piece surfaces would be negligible.
The images are also compared for top concavity C, which is the maximum
depth to which the weld bead 445 has sunken, measured from the top surface
of one of the blanks or the thinner blank 130, if they are of dissimilar
thickness. The reference surface for purposes of the top concavity C
measurement may change based upon the alignment and thickness of the work
piece blanks 130, 180.
The fourth dimensional area selected for purposes of comparison is top
convexity D. Top convexity D is the maximum height of the weld bead 445,
measured from the top surface of the blanks or the top surface of the
thicker blank 180 if blanks of dissimilar thickness are used. Like top
concavity C, the reference surface for purposes of the top convexity D
measurement may change based upon the alignment and thickness of the work
pieces 130, 180.
In an embodiment denoted in FIG. 3a, all images of the weld bead 445 are
generated by the visual sensor 410 from above the work pieces. However, it
should be understood that it is also possible in another embodiment of the
invention, to utilize an additional visual sensor for viewing the portion
of the weld bead 450 formed along the bottom surface of the work pieces
130, 180. Additional dimensional comparison areas may also be included in
the weld bead analysis. These additional comparison areas, as denoted in
FIG. 3a, may include root width E, bottom concavity F, and bottom
convexity G. Further, an additional visual sensor may be incorporated for
use in a dual-cell laser configuration of the present invention so that
more than one segment of the weld bead may be inspected in cooperation
with each of the dual laser weld heads. Root width E is the distance
between the two limit points defining the edges of the weld bead 450,
taken along an axis perpendicular to the length of the seam on the bottom
surface of the work piece.
As illustrated in FIG. 3b, the blanks 130, 180 may be aligned so neither
their respective top nor bottom surfaces lie in the same plane. In this
circumstance, bottom mismatch H will occur as the difference in height
between the bottom surface of one blank, and the bottom surface of the
other blank. Bottom mismatch may also be selected as a dimensional
comparison area.
Referring again to FIG. 3a, bottom concavity F is shown as the maximum
depth of the weld bead 450 below the bottom surface of the work pieces
130, 180. The maximum height of the weld bead 450 as measured from the
bottom surface of the work pieces may also be checked. This measurement is
defined as bottom convexity G.
The selected dimensional comparison steps are described above in an
exemplary sequence representative of the preferred embodiment of the
present invention. However, any sequence of the above steps is equally
satisfactory and the preceding description is presented for purposes of
illustration but not limitation. Moreover, the exemplary procedure setting
forth the comparison and analysis steps is not limited to any particular
number or combination of dimensional areas described above. Additional
inspection steps may be added, and existing steps may be removed without
departing from the scope of the invention.
New reference images may be created that adopt existing dimensional
comparison areas, create new areas, or utilize combinations of both. A
reference image may have adjustable tolerance zones which can be set for
each area of dimensional comparison. In this manner, distinct reference
images can exist for use under particular conditions.
A visual display monitor, such as is well known, may be connected to the
image processing board or any of the control computers 105, or both, to
display the weld bead image captured by the visual sensor 410. An
additional monitor may be connected to display, for example, the reference
image or dimensional comparison area tolerance zones using graphic
overlays of the predetermined parameters. The system may also employ a
communication board to send signals to other equipment, such as, for
example, any of the control computers 105, as dictated by the results of
the weld bead analysis and for purposes of archiving captured images for
future analyses and comparisons.
In the present invention, a clear image of the weld bead is critical.
Therefore, it is necessary to safeguard the visual sensor 410 from
contamination. Although the visual sensor 410 is maintained at a
preselected distance 425 (FIG. 6b) rearward from the laser weld head 355,
the visual sensor 410 is sufficiently proximate the weld pool 402 to be
affected by plasma, weld spatter, and other debris. Plasma, smoke and
particles of liquid metal (spatter) that are emitted from the weld pool
402 during the welding process, may migrate to, and damage the visual
sensor 410. Therefore, a means for preventing such damage is preferably
utilized. In one preferred embodiment of the invention, a compressed air
stream 485 is employed to pass across, and deflect any errant debris away
from the visual sensor. Other methods such as the gas jet 385 used with
the laser weld head 355, filters or vacuum means, for example, may be
applied to perform an equivalent function.
With reference to FIGS. 5 and 11, an exit conveyer station 500 is depicted
as part of the automated welding system 100. The station 500 includes an
exit conveyor 505 that cooperates with the second conveyor 240 to transfer
the welded work piece from the laser welding bed 330 to a removal station
510. The removal station can include another articulating arm robot 520
that is similar in design to and can include any or all of the features of
the robots 120, 175 already described. Depending on whether the welded
work piece has been accepted or rejected during the inspection of the
laser weld, the removal robot 520 will remove the work piece from the exit
conveyer 505 and put it onto one of a plurality of accepted skids 530 or
on a reject skid 540. Since the quantity of rejected parts is likely to be
very small and since it may be desirable to immediately remove and inspect
any rejected work pieces, it may not be necessary to put the rejected
pieces on a skid. In such a case, 540 may be replaced by a gravity roller
conveyor or other means for removing the work piece from the automated
welding system 100. For worker safety, the exit conveyor station 500 is
configured similar to the feeder station 110 and is surrounded with a
safety fence or partition 545 configured with light curtain assemblies or
specially adapted doors, or both, to detect intrusions into the work area
of the exit station 500. Additionally, the accepted skids or the rejected
skids, or both, are surrounded by light curtains 555 having the same
capabilities described with respect to the feeder station 110.
INDUSTRIAL APPLICABILITY
From the foregoing, it can be appreciated that the present invention
fulfills a real but heretofore unmet need for a structurally improved
welded work piece that is less expensive to manufacture, includes fewer
parts, and is lighter in weight. The present invention also fulfills the
need for a method for manufacturing such a welded work piece that
overcomes the undesirable features, deficiencies, and shortcomings of
presently available welded work pieces and methods for their manufacture.
The invention fulfills these needs of the automotive industry through the
novel design of a welding gantry that comprises a laser beam aimed at a
compound angle, to irradiate the pieces to be welded. The welding gantry
may also comprise an inspection device that travels with the laser welder
during the welding operation. The inspection device captures images of the
weld bead and transmits those images to an image processing board. Various
weld bead parameters are then compared to reference values by a
distinction device. If the weld bead is within tolerance on all
parameters, the work piece is considered accepted and moved to an accepted
work piece skid. If the weld bead is not within tolerance on all
parameters, the work piece is considered rejected and moved to a rejected
work piece skid. The welding gantry may also comprise protection means for
the welding head and/or the inspection device, such as gas streams.
The invention also fulfills the needs of the automotive industry for
improved work pieces through the use of an automated system that comprises
robots for the transport of sheet metal blanks to a conveyor system, a
conveyor system, a precision shearing device, a laser welding gantry, and
robots for the transport of welded work pieces from the conveyor.
Each of the described embodiments and variations, as well as other obvious
yet undescribed embodiments of the invention, and equivalents thereof, may
be used either alone or in combination with each of the other embodiments.
While particular preferred embodiments of the invention have been
illustrated and described, various modifications and combinations can be
made without departing from the spirit and scope of the invention, and all
such modifications, combinations, and equivalents are intended to be
covered and claimed.
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